Atmospheric impacts on climatic variability of surface incident solar radiation

The Earth's climate is driven by surface incident solar radiation (R-s). Direct measurements have shown that R-s has undergone significant decadal variations. However, a large fraction of the global land surface is not covered by these observations. Satellite-derived R-s has a good global coverage but is of low accuracy in its depiction of decadal variability. This paper shows that daily to decadal variations of R-s, from both aerosols and cloud properties, can be accurately estimated using globally available measurements of Sunshine Duration (SunDu). In particular, SunDu shows that since the late 1980's R-s has brightened over Europe due to decreases in aerosols but dimmed over China due to their increases. We found that variation of cloud cover determines R-s at a monthly scale but that aerosols determine the variability of R-s at a decadal time scale, in particular, over Europe and China. Because of its global availability and long-term history, SunDu can provide an accurate and continuous proxy record of R-s, filling in values for the blank areas that are not covered by direct measurements. Compared to its direct measurement, R-s from SunDu appears to be less sensitive to instrument replacement and calibration, and shows that the widely reported sharp increase in R-s during the early 1990s in China was a result of instrument replacement. By merging direct measurements collected by Global Energy Budget Archive with those derived from SunDu, we obtained a good coverage of R-s over the Northern Hemisphere. From this data, the average increase of R-s from 1982 to 2008 is estimated to be 0.87W m(-2) per decade.